Dispersible and conductive nano graphene platelets

ABSTRACT

The present invention provides a dispersible and electrically conductive nano graphene platelet (NGP) material comprising at least a single-layer or multiple-layer graphene sheet, wherein the NGP material has an oxygen content no greater than 25% by weight and no less than 5% by weight. This NGP material can be produced by: (a) preparing a pristine NGP material from a graphitic material; and (b) subjecting the pristine NGP material to an oxidation treatment. Alternatively, the production process may comprise: (A) preparing a graphite oxide (GO) from a laminar graphite material; (b) exposing the GO to a first temperature for a first period of time to obtain exfoliated graphite; and (c) exposing the exfoliated graphite to a second temperature in a protective atmosphere for a second period of time. Conductive NGPs can find applications in transparent electrodes for solar cells or flat panel displays, additives for battery and supercapacitor electrodes, conductive nanocomposite for electromagnetic wave interference (EMI) shielding and static charge dissipation, etc.

FIELD OF THE INVENTION

The present invention relates generally to the field of carbon- orgraphite-based nano materials, and more particularly to nano grapheneplatelets (NGPs), including their oxidized versions (graphite oxide nanoplatelets), that are soluble or dispersible in a liquid medium or amatrix material and are electrically conducting.

BACKGROUND OF THE INVENTION

The present discussion of the prior art will make reference to thepatent literature and technical papers listed at the end of thissection.

The nanoscale graphene platelet (NGP) or graphene nano-sheet is anemerging class of nano materials. An NGP is a nanoscale plateletcomposed of one or more layers of a graphene plane, with a plateletthickness from 0.335 nm to 100 nm. In a graphene plane, carbon atomsoccupy a 2-D hexagonal lattice in which carbon atoms are bonded togetherthrough strong in-plane covalent bonds. In the c-axis or thicknessdirection, several graphene planes may be weakly bonded together throughvan der Waals forces to form a multi-layer NGP. An NGP may be viewed asa flattened sheet of a carbon nano-tube (CNT), with a single-layer NGPcorresponding to a single-wall CNT and a multi-layer NGP correspondingto a multi-wall CNT.

For more than six decades, scientists have presumed that a single-layergraphene sheet (one atom thick) could not exist in its free state basedon the reasoning that its planar structure would be thermodynamicallyunstable. Somewhat surprisingly, several groups worldwide have recentlysucceeded in obtaining isolated graphene sheets [Refs. 1-9]. NGPs arepredicted to have a range of unusual physical, chemical, and mechanicalproperties. Several unique properties associated with these 2-D crystalshave been discovered. In addition to single graphene sheets,double-layer or multiple-layer graphene sheets also exhibit unique anduseful behaviors. In the present context, single-layer andmultiple-layer graphene sheet structures are collectively referred to asNGPs. Graphene platelets may be oxidized to various extents during theirpreparation, resulting in graphite oxide (GO) platelets. Hence, althoughNGPs preferably or primarily refer to those containing no or low oxygencontent, they can include GO nano platelets of various oxygen contents.

Although practical electronic device applications for graphene are notenvisioned to occur within the next 5-10 years, its application as anano filler in a composite material is imminent. However, theavailability of processable graphene sheets in large quantities isessential to the success in exploiting composite and other applicationsfor graphene. The present patent application addresses issues related tothe production of processable or dispersible NGPs.

The processes for producing NGPs and NGP nanocomposites have beenrecently reviewed by the applicants, Jang and Zhamu [Ref. 9]. Basically,there are four different approaches that have been followed to produceNGPs. Their advantages and shortcomings are briefly summarized asfollows:

Approach 1: Formation and Reduction of Graphite Oxide (GO) Platelets

The first approach entails treating a laminar graphite material (e.g.,in most cases, natural graphite powder) with an intercalant and anoxidant (e.g., concentrated sulfuric acid and nitric acid, respectively)to obtain a graphite intercalation compound (GIC) or, actually, graphiteoxide (GO). The obtained GIC or GO is then subjected to exfoliationusing either a thermal shock exposure or a solution-based graphene layerseparation approach.

Technically, the acid-treated graphite is actually oxidized graphite orgraphite oxide (GO), rather than pristine graphite. In the thermal shockexposure approach, the GIC or GO is exposed to a high temperature(typically 800-1,050° C.) for a short period of time (typically 15 to 60seconds) to exfoliate the treated graphite. Typically, the exfoliatedgraphite oxide is then subjected to a further sheet or flake separationtreatment using air milling, mechanical shearing, or ultrasonication ina liquid (e.g., water).

In the solution-based graphene separation approach, the GO powder isdispersed in water or aqueous alcohol solution, which is subjected toultrasonication. Alternatively, the GO powder dispersed in water issubjected to some kind of ion exchange or purification procedure in sucha manner that the repulsive forces between ions residing in theinter-planar spaces overcome the inter-graphene van der Waals forces,resulting in graphene layer separations.

In both the heat- or solution-induced exfoliation approaches, theresulting products are GO platelets that must undergo a further chemicalreduction treatment to reduce (but normally not eliminate) the oxygencontent. Typically even after reduction, the electrical conductivity ofGO platelets remains much lower than that of pristine graphene.Furthermore, the reduction procedure often involves the utilization ofundesirable chemicals, such as hydrazine. In some cases ofsolution-based exfoliation, the separated and dried GO platelets werere-dispersed in water and then cast into thin GO films. These films wereexposed to a high temperature, high vacuum environment forde-oxygenation, but the resulting GO platelets were no longerdispersible in water or other solvents.

Examples of Approach 1 are briefly discussed below:

-   -   (a) Bunnell [10-12] developed a method in late 1988 that        entailed intercalating graphite with a strong acid to obtain a        GIC, thermally exfoliating the GIC to obtain discrete layers of        graphite, and then subjecting the graphite layers to ultrasonic        energy, mechanical shear forces, or freezing to separate the        layers into discrete flakes. Although flakes as small as 10 nm        were cited in the report [12], most of the flakes presented in        the examples appeared to be thicker than 100 nm.    -   (b) In a similar manner, Zaleski, et al. [13] used air milling        to further delaminate thermally exfoliated graphite flakes. The        resulting structures exhibited a specific surface area of 35        m²/g, corresponding to an average flake thickness of        approximately 25 nm.    -   (c) Horiuchi, Hirata, and co-workers [14-19] prepared        nano-scaled graphite oxide (GO) platelets, which they coined as        carbon nano-films. These films were prepared by a two-step        process—oxidation of graphite and purification of the resulting        graphite oxide. The oxidation of graphite was conducted using        the now well-known Hummer's method [20, 21], which entailed        immersing natural graphite particles in a mixture of H₂SO₄,        NaNO₃, and KMnO₄ to obtain GICs that actually were GOs. By        hydrolyzing the GIC, functional groups, such as acidic hydroxyl        groups and ether groups, were introduced into the inter-graphene        layer spaces. Each of the graphite oxide layers became a        multiple-charge anion, having a thickness of approximately 0.6        nm. When the excess small ions derived from the oxidants (e.g.,        NaNO₃, and KMnO₄) were thoroughly removed by a purification        process, many layers tended to automatically separate from each        other due to interlayer electrostatic repulsion. The resulting        GO layers formed a stable dispersion in water. According to        Horiuchi, et al. [14], single-layer graphene was detected.    -   (d) It may be noted that the approach of using electrostatic        repulsion to separate graphene oxide layers was pursued earlier        in 1998 by Liu and Gong [22], as a first step in their attempt        to synthesize polyaniline-intercalated GO. In a 3-D graphite        crystal, the inter-layer spacing (L_(d)) is 0.335 nm, which is        known to increase to 0.6-1.1 nm if graphite is oxidized to        produce GO. Further, GO is hydrophilic and can be readily        dispersed in aqueous solution.    -   (e) Dekany et al. [23] observed that the inter-graphene spacing        in GO was increased to L_(d)=1.23 nm when GO particles were        dispersed in 0.05 N NaOH solution. When dispersed in a 0.01 N        NaOH solution, the spacing was essentially infinite, likely        implying that GO was completely exfoliated to become a        disordered structure.    -   (f) Chen et al. [24] exposed GO to a temperature of 1,050° C.        for 15 seconds to obtain exfoliated graphite, which was then        subjected to ultrasonic irradiation in a mixture solution of        water and alcohol.    -   (g) Jang et al. [25] thermally expanded GIC or graphite oxide to        produce exfoliated graphite and subjected exfoliated graphite to        mechanical shearing treatments, such as ball milling, to obtain        partially oxidized NGPs.    -   (h) Thermal exfoliation as a way of producing nano-structured        graphite was also attempted by Petrik [26].    -   (i) Thermal exfoliation of intercalated graphite or graphite        oxide was conducted by Drzal et al. [27] using microwaves as a        heat source.    -   (j) Aksay, McAllister, and co-workers [7-9, 66] also used        thermal exfoliation of GO to obtain exfoliated graphite oxide        platelets, which were found to contain a high proportion of        single-layer graphene oxide sheets, based on the BET method with        nitrogen gas adsorption in the dry state and in an ethanol        suspension with methylene blue dye as a probe.    -   (k) Several polar organic compounds and polymers have been        intercalated into inter-graphene or inter-flake spaces to form        intercalated or exfoliated GO nanocomposites. These include poly        (vinyl alcohol) [28-30], poly (acrylamide) [31], and poly        (acrylic acid) [32]. Intercalation of hydrophobic polymers, such        as poly (vinyl acetate) [33], into GO was also achieved by in        situ polymerization. Partial reduction of a polymer-GO to a        polymer-graphene nanocomposite also could be accomplished        electrochemically or chemically [22, 34-37].    -   (l) Preparation of ultra-thin films by a layer-by-layer        self-assembly approach from GO nano platelets and polymer        electrolytes also has been investigated [38-44]. Although the        original intent of these studies was primarily to fabricate        self-assembled GO-poly (ethylene oxide) nanocomposites, their        first step almost always involved exfoliation and separation of        GO platelets. This was evidenced by the X-ray diffraction data        of the resulting structures that showed complete disappearance        of those diffraction peaks corresponding to graphite oxide or        pristine graphite [38, 40].    -   (m) Stankovich et al. [45] followed the approaches of Hirata et        al. [17-19] to produce and disperse graphite oxide sheets in        water to obtain stable colloidal dispersions. The graphite oxide        dispersion was then reduced with hydrazine, a procedure        previously used by Liu and Gong earlier [22], but in the        presence of poly (sodium 4-styrenesulfonate). This process led        to the formation of a stable aqueous dispersion of        polymer-coated graphene platelets. Stankovich et al. [46]        further developed a method to produce less hydrophilic GO        platelets using an isocyanate treatment. However, unless        stabilized by selected polymers, the chemically modified        graphene sheets obtained through these methods tend to        precipitate as irreversible agglomerates due to their        hydrophobic nature. The resulting agglomerates became insoluble        in water and organic solvents.    -   (n) Li et al. [47] overcame this issue by using ammonium to        adjust the pH value of a dispersion of chemically modified        graphene sheets in water, which served to maximize the charge        density on the resulting graphene sheets. The resulting        electrostatic forces acted to stabilize the aqueous suspension.    -   (o) Si and Samulski [48] reported a chemical route to aqueous        solutions of isolated graphene sheets by reducing graphene oxide        in three steps. (1) pre-reduction of graphene oxide with sodium        borohydride at 80° C. for 1 h to remove the majority of the        oxygen functionality; (2) sulfonation with the aryl diazonium        salt of sulfanilic acid in an ice bath for 2 h; and (3)        post-reduction with hydrazine (100° C. for 24 h) to remove any        remaining oxygen functionality. The lightly sulfonated graphene        can be readily dispersed in water at reasonable concentrations        (2 mg/mL) in the pH range of 3-10. Isolated graphene sheets        persist in the mixture of water and organic solvents including        methanol, acetone, acetonitrile, thus making it possible to        further modify its surface for applications such as        reinforcements in composites. This is a very tedious process,        nevertheless.    -   (p) Another very tedious process for the preparation of GO nano        sheets, proposed by Becerril, et al. [67], entailed (1)        intercalating-oxidizing graphite with a solution of NaNO₃ and        KMnO₄ in concentrated H₂SO₄ for 5 days; (2) washing the oxidized        graphite with 5 wt. % H₂SO₄ in water and reacting the washed        oxidized graphite with a 30 wt. % aqueous solution of H₂O₂ to        complete the oxidation; (3) removing inorganic anions and other        impurities through 15 washing cycles that included        centrifugation, discarding supernatant liquid, and re-suspending        the solid in an aqueous mixture of 3 wt. % H₂SO₄ and 0.5 wt. %        H₂O₂ using stirring and ultrasonication; (4) carrying out        another set of centrifugation and washing procedures three times        using 3 wt % HCl in water as the dispersion medium and then one        more time using purified water to re-suspend the solid; (5)        passing this suspension through a weak basic ion-exchange resin        to remove remaining acid; and (6) drying the suspension to        obtain a powder.        Approach 2: Direct Formation of Pristine Nano Graphene Platelets    -   (q) Without going through a chemical intercalation route,        Mazurkiewicz [49] claimed to have produced graphite nano        platelets having an average thickness in the range of 1-100 nm        through high-pressure milling of natural flake graphite.        However, no evidence was presented [49] to show that truly thin        platelets (e.g., those <10 nm in thickness) were produced.    -   (r) Shioyama [50] prepared a potassium-intercalated GIC from        highly oriented pyrolytic graphite (HOPG), initiated in situ        polymerization of isoprene or styrene in the inter-graphene        spaces, and then thermally decomposed inter-graphene polymer        chains at a high temperature (500-1,000° C.). The volatile gas        molecules served to exfoliate graphite layers, and, after the        volatile gas escaped, isolated graphene sheets were obtained.        Unfortunately, Shioyama did not discuss the thickness of the        isolated graphene sheets.    -   (s) Jang, et al. [3,4] succeeded in isolating single-layer and        multi-layer graphene structures from partially carbonized or        graphitized polymeric carbons, which were obtained from a        polymer or pitch precursor. Carbonization involves linking        aromatic molecules or planar cyclic chains to form graphene        domains or islands in an essentially amorphous carbon matrix.        For instance, polymeric carbon fibers were obtained by        carbonizing polyacrylonitrile (PAN) fibers to a desired extent        that the fiber was composed of individual graphene sheets        isolated or separated from each other by an amorphous carbon        matrix. The resulting fibers were then subjected to a solvent        extraction, or intercalation/exfoliation treatment. Graphene        platelets were then extracted from these fibers using a ball        milling procedure.    -   (t) Mack, Viculis, and co-workers [51, 52] developed a        low-temperature process that involved intercalating graphite        with potassium melt and contacting the resulting K-intercalated        graphite with alcohol, producing violently exfoliated graphite        containing many ultra-thin NGPs. The process must be carefully        conducted in a vacuum or an extremely dry glove box environment        since pure alkali metals, such as potassium and sodium, are        extremely sensitive to moisture and pose an explosion danger. It        is questionable if this process is easily amenable to the mass        production of nano-scaled platelets. One major advantage of this        process is the notion that it produces non-oxidized graphene        sheets since no acid/oxidizer intercalation or a high        temperature is involved.    -   (u) In 2004, Novoselov, Geim, and co-workers [1,2] prepared        single-sheet graphene by removing graphene from a graphite        sample one sheet at a time using a “Scotch-tape” method.        Although this method is not amenable to large-scale production        of NGPs, their work did spur globally increasing interest in        nano graphene materials, mostly motivated by the thoughts that        graphene could be useful for developing novel electronic        devices.    -   (v) Zhamu and Jang [75] developed a very effective way of        exfoliating/separating NGPs from natural graphite and other        laminar graphitic materials by exposing the material (without        any intercalation or oxidation) to an ultrasonication treatment.        This process may be considered as peeling off graphene layers at        a rate of 20,000 layers per second (if the ultrasonic frequency        is 20 kHz) or higher (if higher frequency). The resulting NGPs        are pristine graphene without any intentionally added or bonded        oxygen.        Approach 3: Epitaxial Growth and Chemical Vapor Deposition of        Nano Graphene Sheets on Inorganic Crystal Surfaces    -   (w) Small-scale production of ultra-thin graphene sheets on a        substrate can be obtained by thermal decomposition-based        epitaxial growth [53] and a laser desorption-ionization        technique [54]. A scanning probe microscope was used by Roy et        al. [55] and by Lu et al. [56] to manipulate graphene layers at        the step edges of graphite and etched HOPG, respectively, with        the goal of fabricating ultra-thin nano-structures. It was not        clear if single graphene sheets were obtained using this        technique by either group. Epitaxial films of graphite with only        one or a few atomic layers are of technological and scientific        significance due to their peculiar characteristics and great        potential as a device substrate [57-63]. The graphene sheets        produced are meant to be used for future nano-electronic        applications, rather than composite reinforcements.        Approach 4: The Bottom-Up Approach (Synthesis of Graphene from        Small Molecules)    -   (x) X. Yang, et al. [65] synthesized nano graphene sheets with        lengths of up to 12 nm using a method that began with        Suzuki-Miyaura coupling of        1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic        acid. The resulting hexaphenylbenzene derivative was further        derivatized and ring-fused into small graphene sheets. This is a        slow process that thus far has produced very small graphene        sheets.

There are several major issues associated with the aforementionedprocesses and the resulting products:

-   (1) The GO nano platelets prepared by Approach 1, albeit dispersible    in water and several other polar liquids such as ethanol and    acetone, are not dispersible in a wide range of organic solvents.-   (2) The GO nano platelets exhibit an electrical conductivity    typically several orders of magnitude lower than the conductivity of    pristine NGPs. Even after chemical reduction, the GO still exhibits    a much lower conductivity than pristine NGPs. It appears that the    preparation of intercalated graphite, which involves the oxidizing    agent such as nitric acid or potassium permanganate, typically and    necessarily requires graphite to be heavily oxidized. Complete    reduction of these highly oxidized graphite platelets hitherto has    not been successfully attained.-   (3) The GO nano platelets, after a high degree of chemical    reduction, are able to recover some of the properties of pristine    graphite, but are typically no longer dispersible in water and most    of the organic solvents.-   (4) The NGPs produced by Approach 2 and Approach 3 are normally    pristine graphene and highly conducting. However, pristine NGPs,    just like reduced GO platelets, are typically not soluble or    dispersible in water or other organic solvents. It is also difficult    to homogeneously mix or disperse pristine NGPs in a polymer matrix.    These features make it difficult to fabricate nanocomposite parts    with good filler dispersion or good filler-matrix interfacial    bonding, which are essential to the realization of good composite    properties.

Hence, it is an object of the present invention to provide a nanographene platelet material that is soluble or dispersible in a range oforganic solvents while maintaining good properties of pristine graphene(e.g., good electrical or thermal conductivity).

It is another object of the present invention to provide a processablenano graphene platelet material that can be dispersed in a range ofpolymer matrices to form nanocomposites of desirable properties (e.g.,achieving good electrical, thermal, or mechanical properties).

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SUMMARY OF THE INVENTION

The present invention provides a dispersible and electrically conductivenano graphene platelet (NGP) material comprising at least a single-layeror multiple-layer graphene sheet, wherein the NGP material has an oxygencontent no greater than 25% by weight and no less than 5% by weight. Theelectrical conductivity of NGPs in the present context was measuredafter the NGPs were formed into a thin film of approximately 100 nm inthickness or less. The term “dispersible” means NGPs being dispersibleor soluble in a liquid solvent, either a polar solvent (such as water orethanol) or a more hydrophobic or non-polar solvent (such as benzene,toluene, or NMP).

This NGP material can be produced by two different sets of approaches:The first set includes: (a) preparing a pristine NGP material from agraphitic material; and (b) subjecting the pristine NGP material to anoxidation treatment. The second set includes: (A) preparing a graphiteoxide (GO) from a laminar graphite material; (B) exposing the GO to afirst temperature for a first period of time to obtain exfoliatedgraphite (a step called “exfoliation”); and (C) exposing the exfoliatedgraphite to a second temperature in a protective atmosphere for a secondperiod of time (a step called “de-oxygenation”). This process does notrequire a chemical reduction procedure.

In one preferred embodiment (the first set of approaches), the materialmay be produced from a process that comprises: (a) preparing a pristineNGP material from a graphitic material; and (b) subjecting the pristineNGP material to an oxidation treatment that attaches oxygen-containingfunctional groups to the NGP. The resulting NGP material has an oxygencontent no greater than 25% by weight. Preferably, the NGP material hasan oxygen content no less than 5% by weight. A most preferred oxygencontent range is from approximately 10% to 20% by weight.

The starting graphitic material is selected from the group consisting ofnatural graphite, artificial graphite, graphite oxide, graphitefluoride, graphite fiber, carbon fiber, carbon nano-fiber, carbonnano-tube, mesophase carbon micro-bead (MCMB) or carbonaceousmicro-sphere (CMS), soft carbon, hard carbon, and combinations thereof.The pristine NGP material is preferably produced by a process comprisinga procedure selected from: (A) intercalating the graphitic material witha non-oxidizing agent, followed by a thermal or chemical exfoliationtreatment in a non-oxidizing environment; (B) subjecting the graphiticmaterial to a supercritical fluid environment for inter-graphene layerpenetration and exfoliation; or (C) dispersing the graphitic material ina powder form to an aqueous solution containing a surfactant ordispersing agent to obtain a suspension and subjecting the suspension todirect ultrasonication; or a combination thereof. Any of these threeprocedures can lead to the production of pristine or un-oxidized NGPs.

In Procedure (A), a particularly preferred step comprises (i)intercalating the graphitic material with a non-oxidizing agent,selected from an alkali metal (e.g., potassium, sodium, lithium, orcesium), alkaline earth metal, or an alloy, mixture, or eutectic of analkali or alkaline metal; and (ii) a chemical exfoliation treatment(e.g., by immersing K-intercalated graphite in ethanol solution).

In Procedure (B), a preferred step comprises immersing the graphiticmaterial to a supercritical fluid, such as carbon dioxide (e.g., attemperature T>31° C. and pressure P>7.4 MPa) and water (e.g., at T>374°C. and P>22.1 MPa), for a period of time sufficient for inter-graphenelayer penetration (tentative intercalation). This step is then followedby a sudden de-pressurization to exfoliate individual graphene layers.Other suitable supercritical fluids include methane, ethane, ethylene,hydrogen peroxide, ozone, water oxidation (water containing a highconcentration of dissolved oxygen), or a mixture thereof.

In Procedure (C), a preferred step comprises (a) dispersing particles ofa graphitic material in a liquid medium containing therein a surfactantor dispersing agent to obtain a suspension or slurry; and (b) exposingthe suspension or slurry to ultrasonic waves (a process commonlyreferred to as ultrasonication) at an energy level for a sufficientlength of time to produce the separated nano-scaled platelets, which arepristine, non-oxidized NGPs.

It may be noted that although Kaschak, et al. [76] proposed a method ofmodifying graphite by introducing a supercritical fluid into intersticesof graphite flake, the graphite flake used was intercalated orintercalated/oxidized graphite (rather than the original naturalgraphite). The interstices of intercalated/oxidized graphite had beenexpanded and chemically modified due to the presence of intercalantspecies (such as sulfuric acid) or oxidation-induced functional groups(such as carboxyl). Kaschak, et al. did not teach the approach ofdirectly intercalating natural (un-treated) graphite flake with asupercritical fluid; nor did they teach the approach of intercalatingand exfoliating graphite using the same supercritical fluid. Themodified graphite as proposed by Kaschak, et al. [76] still required ahigh temperature exposure step, typically at 700-1,200° C., to exfoliatethe intercalated and modified graphite. Furthermore, they really did notprovide any evidence to show the existence of nano-scaled graphiteparticles that they claimed they produced with this method. Inparticular, they claimed that “one advantage of the invention is thatthe aforementioned methods may be used to manufacture graphite in a formthat has a thickness of less than about 10 microns, preferably less thanabout 1 micron, more preferably less than about 100 nm, even morepreferably less than about 10 nm, and most preferably less than about 1nm.” However, they did not fairly suggest the conditions under whichgraphite particles with a thickness less than 10 mm or 1 nm could beproduced. This was truly a big claim and should have been supported bysolid experimental evidence; unfortunately, absolutely no evidencewhatsoever was presented.

Gulari, et al. [77] proposed a method of delaminating a graphitestructure with a coating agent solubilized in a supercritical fluid. Thecoating agent was a polymer, monomer, or oil, according to Gulari, etal. The method comprises diffusing a coating agent in a supercriticalfluid between layered particles of a graphite structure andcatastrophically depressurizing the supercritical fluid to delaminatethe coated graphite particles. However, Gultari, et al. [76] failed tomention anything about the thickness of the delaminated particles. Itwas not clear if and how graphite platelets with a thickness less than100 nm could be produced with this method. Gulari, et al. presumed thata coating agent was needed to prevent the reformation of the covalentbands between graphite particles when they were broken duringdelamination. This is rather confusing or misleading since it iswell-known that the bonding between graphite layers is van der Waalsforce rather than covalent bond. Furthermore, a coating agent isproblematic if a pure graphene product is desired. Gulari, et al. [77]did not teach if a supercritical fluid without a coating agentsolubilized therein would be capable of delaminating graphite layers.

Neither Kaschak, et al. [76] nor Gulari, et al. [77] mentioned anythingabout the properties of the resulting exfoliated graphite. It was notclear if their graphite products were conductive or soluble in asolvent.

By contrast, after an intensive research and development effort, we havefound that a supercritical fluid, alone without a coating agent, wascapable of both intercalating and exfoliating a graphitic materialwithout involving an additional intercalation or oxidation step (asrequired in Kaschak, et al. [76]). Further, this supercriticalfluid-based process is capable of producing nano graphene platelets thatare ultra-thin (<10 nm) and, in many cases, thinner than 1 nm.

As a second step of the presently invented process, the oxidationtreatment comprises subjecting the pristine NGP material to an oxidizingagent preferably selected from ozone, sulfonic (SO₃) vapor, anoxygen-containing gas, hydrogen peroxide vapor, nitric acid vapor, or acombination thereof. Preferably, the treatment comprises subjecting thepristine NGP material to an oxidizing agent in a hydrogen-containingenvironment. Although oxidation treatment can be conducted by immersingNGPs in a liquid acid and/or oxidizer environment, such a procedurerequires a subsequent water-rinsing and purification step (such arinsing procedure is not as tedious as required in the case ofconventional sulfuric acid-intercalation graphite, nevertheless). Hence,a gaseous treatment requiring no post-treatment rinsing is preferred.

A primary goal of the oxidation treatment is to impart solubility ordispersibility to the obtained pristine NGPs without a significantcompromise in electrical conductivity. After an extensive and in-depthstudy we have come to discover that dispersible and conductive NGPs canbe produced with an NGP oxygen content no greater than 25% by weight,preferably below 20% by weight, further preferably between 5% and 20% byweight. The oxygen content can be determined using chemical elementalanalysis and/or X-ray photoelectron spectroscopy (XPS).

For the purpose of facilitating discussion, we may define those graphiteplatelets having an oxygen content higher than 15% by weight as GO nanoplatelets and those lower than approximately 15% as nano graphene. Thepristine nano graphene refers to those NGPs that have an oxygen contentless than 1% by weight. Hence, the present process provides GO (>15% bywt. 0), nano graphene (<15% by wt. 0), and pristine nano graphene (<1%by wt. 0).

The NGPs or GO platelets prepared with this process, when formeddirectly into a thin film with a thickness no greater than 100 nm,typically maintain an electrical conductivity of at least 100 S/cm and,in many cases, greater than 1,000 S/cm. With an oxygen content nogreater than 25% by weight, typically the NGP thin film exhibits anelectrical conductivity no less than 1 S/cm.

The graphitic material, for the preparation of pristine NGPs, may beselected from the group consisting of natural graphite, artificialgraphite (e.g., highly oriented pyrolytic graphite, HOPG), graphiteoxide, graphite fluoride, graphite fiber, carbon fiber, carbonnano-fiber, carbon nano-tube, mesophase carbon micro-bead (MCMB) orcarbonaceous micro-sphere (CMS), graphitized soft carbon, hard carbon,and combinations thereof. MCMBs or CMS are usually obtained from apetroleum heavy oil or pitch, coal tar pitch, or polynuclear hydrocarbonmaterial (highly aromatic molecules). When such a precursor pitchmaterial is carbonized by heat treatment at 400° to 550°, micro-crystalscalled mesophase micro-spheres are formed in a non-crystalline pitchmatrix. These mesophase micro-spheres, after being isolated from thepitch matrix (which is typically soluble in selected solvents), areoften referred to as meso-carbon micro-beads (MCMB). The MCMBscommercially available are those that have been subjected to a furtherheat treatment at a temperature in the range of 2,000° C. and 3,000° C.

In many cases, the NGPs produced in our studies have a specific surfacearea in the range of approximately 300 m²/g to 2,600 m²/g. The NGPsobtained with the presently invented process tend to contain asignificant proportion of single-layer graphene (with a thickness of0.34-0.4 nm) or graphene of few layers (<2 nm).

The resulting NGPs prepared according to the presently invented process,although having a minimal amount of oxygen-containing groups (hence,remain highly conducting), become soluble or dispersible in water andseveral other organic solvents, such as methanol, ethanol, acetone, NMP,and toluene. These NGPs can be further functionalized by carrying out anadditional step of contacting the lightly oxidized NGP obtained in step(b) with a reactant such that a functional group is added to a surfaceor edge of the nano graphene platelet, wherein the functional group isselected from alkyl or aryl silane, alkyl or aralkyl group, hydroxylgroup, amine group, fluorocarbon, or a combination thereof.

As indicated earlier, for practical purposes (e.g., for the purpose offacilitating discussion), the nano GO platelets that contain less thanapproximately 15% by weight of oxygen (hence, the electricalconductivity of a thin film made from these platelets is typicallygreater than 10 S/cm) are considered nano graphene platelets (NGPs).

In the second set of approaches, the dispersible and conductive NGPmaterial may be produced by (a) preparing a graphite intercalationcompound (GIC) or graphite oxide (GO) from a laminar graphite material;(b) exposing the GIC or GO to a first temperature for a first period oftime to obtain exfoliated graphite; and (c) exposing the exfoliatedgraphite to a second temperature in a protective atmosphere for a secondperiod of time to obtain the desired dispersible nano graphene plateletwith an oxygen content no greater than 25% by weight, preferably below20% by weight, further preferably between 5% and 20% by weight. It maybe noted that the “exfoliated graphite” after step (b) typically has anoxygen content of greater than 25% by weight, based on chemicalelemental analysis and/or X-ray photoelectron spectroscopy (XPS). Hence,the “exfoliated graphite” at this stage comprises primarily graphiteoxide (GO), which needs to be de-oxygenated to a desired extent forachieving both dispersibility and conductivity.

Preferably, the protective atmosphere comprises an inert gas (e.g.,argon), nitrogen, hydrogen, a combination of nitrogen and/or hydrogenwith an inert gas, or vacuum. The first temperature, hereinafter alsoreferred to as an exfoliation temperature, is preferably betweenapproximately 200° C. and 1,500° C., more preferably betweenapproximately 800° C. and 1,300° C., and further preferably at least1,000° C. In one preferred embodiment, the second temperature is atleast 1,000° C. and the second period of time is at least 10 minutes.The second temperature is hereinafter also referred to as ade-oxygenation temperature. In another preferred embodiment, the secondtemperature is at least 1,100° C. and the second period of time is atleast 5 minutes. In still another preferred embodiment, the secondtemperature is at least 1,200° C. and the second period of time is atleast 2 minutes.

The step of preparing a graphite intercalation compound (GIC) orgraphite oxide (GO) comprises subjecting the laminar graphite materialto an acid and/or an oxidizer selected from sulfuric acid, nitric acid,carboxylic acid, sodium or potassium nitrate, KMnO₄, sodium or potassiumchlorate, hydrogen peroxide (H₂O₂), or a combination thereof.

The resulting NGPs prepared according to the presently invented process,although having a minimal amount of oxygen-containing groups, remainsoluble or dispersible in water and several other organic solvents, suchas methanol, ethanol, acetone, NMP, and toluene. These NGPs can befurther functionalized by carrying out an additional step of contactingthe NGP obtained in step (c) with a reactant such that a functionalgroup is added to a surface or edge of the nano graphene platelet,wherein the functional group is selected from alkyl or aryl silane,alkyl or aralkyl group, hydroxyl group, amine group, fluorocarbon, or acombination thereof.

The presently invented processes are superior to many prior artprocesses (e.g., those belonging to the aforementioned Approach 1 andApproach 2) in several aspects:

-   (1) For instance, as discussed earlier, Aksay, McAllister, and    co-workers [Refs. 7-9, 66] used thermal exfoliation of GO to obtain    exfoliated graphite oxide platelets. The process involved    essentially an exfoliation step (e.g., at 1,050° C. for 30 seconds    or in a propane torch for less than 15 seconds). Such a heat    exposure typically produces graphite oxide platelets (rather than    nano graphene) that, albeit dispersible, are typically not    electrically conducting. Furthermore, this prior art process did not    have a good control over the oxygen content of the resulting GO    platelets.-   (2) In another commonly used prior art approach, as practiced by    Stankovich et al. [45] and Hirata et al. [17-19], graphite was    heavily oxidized to obtain graphite oxide, which was then mixed with    water. The resulting suspension was then subjected to    ultrasonication for an extended period of time to produce colloidal    dispersions of GO platelets. The graphite oxide dispersion was then    reduced with hydrazine, in the presence of poly (sodium    4-styrenesulfonate). This process led to the formation of a stable    aqueous dispersion of polymer-coated graphene platelets. In some    applications, a polymer coating may be undesirable (pure graphene    being preferred). Furthermore, the reducing agent, hydrazine, is a    toxic substance.-   (3) Stankovich et al. [46] further developed a method to produce    less hydrophilic GO platelets using an isocyanate treatment.    However, unless stabilized by selected polymers, the chemically    modified graphene sheets obtained through these methods tend to    precipitate as irreversible agglomerates due to their hydrophobic    nature. The resulting agglomerates became insoluble in water and    organic solvents. By contrast, the presently invented process    provides a convenient approach to the preparation of soluble or    dispersible nano graphene that requires only slight oxidation.-   (4) Becerril, et al [67] and Wang, et al. [68] independently    developed a very similar process for producing transparent, yet    conducting electrode. The electrode was made by following a very    tedious process that involves oxidation of natural graphite to form    GO, repeated washing, ultrasonication, and 15 cycles of impurity    removal steps that include centrifugation, discarding supernatant    liquid, and re-suspending the solid in an aqueous mixture of    sulfuric acid and hydrogen peroxide [67]. The suspension was    eventually spin-coated on a solid substrate to form a GO thin film,    which was then partially reduced by heating the film in a high    vacuum at a high temperature for a long period of time. Such a long    process does not appear to be amenable to mass production of    conducting nano graphene platelets. It may be noted that both    Becerril, et al [67] and Wang, et al. [68], did subject the GO films    to a high temperature treatment after the tedious solution process    for producing GO nano sheets and obtained electrical conductivity as    high as 550 S/cm. However, once such a high temperature treatment    was done, the GO nano sheets were no longer dispersible in water.-   (5) Another unexpected benefit of the presently invented process is    the observation that pristine NGPs remain relatively defect-free on    the basal plane (graphene plane). In contrast, the heavily oxidized    GO platelets are typically highly defected and could not fully    recover the perfect graphene structure even after chemical    reduction. Therefore, the resulting platelets exhibit a conductivity    lower than that of a more perfect NGP obtained from oxidation of a    pristine NGP material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Electrical conductivity data of the thin films made from GO nanoplatelets after various periods of de-oxygenation time at 1,000° C. and1,100° C., respectively.

FIG. 2 Electrical conductivity data plotted as a function of thecorresponding oxygen content in GO nano platelets prepared at twode-oxygenation temperatures.

FIG. 3 Electrical conductivity data of GO nano platelet films aftervarious periods of platelet de-oxygenation time at 1,200° C. and 1,350°C.

FIG. 4 Electrical conductivity data of pristine NGPs and their partiallyoxidized versions prepared according several preferred embodiments ofthe present invention. The data shown in FIG. 2 were herein repeated forthe purpose of comparison.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides soluble or dispersible nano grapheneplatelet (NGP) materials that are also highly conducting. The electricalconductivity of NGPs in the present context was measured after the NGPswere formed into a thin film approximately 100 nm in thickness.

In one preferred embodiment, the NGP material is produced by (a)preparing a pristine NGP material from a graphitic material; and (b)subjecting the pristine NGP material to an oxidation treatment so thatan oxygen content of between 5% and 25% by weight inclusive may beimparted to the NGP. The oxygen atoms presumably exist in functionalgroups such as carboxylic and hydroxyl groups. Preferably, the NGPmaterial has an oxygen content no less than 5% by weight. A particularlyuseful oxygen content range is from approximately 10% to 20% by weightinclusive.

Preparation of Pristine NGPs

The pristine NGP material is preferably produced by a process comprisinga procedure selected from: (A) intercalating the graphitic material witha non-oxidizing agent, followed by a thermal or chemical exfoliationtreatment in a non-oxidizing environment; (B) subjecting the graphiticmaterial to a supercritical fluid environment for inter-graphene layerpenetration and exfoliation; or (C) dispersing the graphitic material ina powder form to an aqueous solution containing a surfactant ordispersing agent to obtain a suspension and subjecting said suspensionto direct ultrasonication. Any one of these three procedures will leadto the production of pristine or un-oxidized NGPs. Preferred modes ofpracticing these three procedures are discussed in more detail asfollows:

Procedure (A):

In Procedure (A), a particularly preferred step comprises (i)intercalating the graphitic material with a non-oxidizing agent,selected from an alkali metal (e.g., potassium, sodium, lithium, orcesium), alkaline metal, or an alloy, mixture, or eutectic of an alkalior alkaline earth metal; and (ii) a chemical exfoliation treatment(e.g., by immersing K-intercalated graphite in ethanol solution).

In addition to alkali metals (e.g. Li, Na, K, Rb, Cs) and alkaline earthmetals (e.g. Mg, Ca, Sr, Ba), elements such as Eu, Yb, Ti, and halogen(Cl, F, I, etc.) can be used to intercalate the starting graphiticmaterial. Intercalation of these elements can be carried out by severaldifferent routes. First, these elements can be intercalatedelectrochemically using a non-aqueous solvent. Second, an alkali plusnaphthalene or benzophenone can be used with a suitable non-aqueoussolvent (e.g., tetrahydrofuran). Third, any of the aforementioned metalscan be intercalated by dissolving in a liquid ammonia solution to createsolvated ions. Fourth, lithium can be intercalated by using n-butyllithium in a hydrocarbon solvent (e.g., hexane). Fifth, element, such asK, or an eutectic of K, can be heated above its melting or eutecticpoint, enabling the melt to intercalate into inter-graphene spaces. Six,the graphitic material can be exposed to a halogen element or halogencompound sealed in a vessel or a two-chamber vessel (one chambercontaining the graphitic material in a fine powder form and the othercontaining the halogen). The first five approaches were mentioned inMack, et al. [51, 52].

For instance, natural flake graphite can be heated to 200° C. in anevacuated container in the presence of potassium to form a first stageintercalation compound. By immersing this intercalation compound inethanol, graphite is exfoliated with resulting graphene sheets dispersedin ethanol. Lithium can be intercalated at higher temperatures and/orpressures. Intercalation using the alkaline earth (Ca, Ba, Sr) orlanthanide metals (Eu, Yb, Sm, Tm) also requires high temperatures andlong reaction times. Any solvent that contains water can be used forexfoliation, including organic solvents that have not been thoroughlydried. This includes water, alcohols, or other hydroxylic solvents(including carboxylic acids), or any combination thereof. Although Mack,et al. [51, 52] prepared NGPs using the alkali metal intercalationapproach, they did not teach about modifying NGPs for solubility, nordid they measure the electrical conductivity of NGPs.

Procedure (B):

Procedure (B) involves delaminating a graphitic material with asupercritical fluid or, analogous to a prior art approach, with acoating agent solubilized in a supercritical fluid. It is known that, ifa substance is heated above its critical temperature (Tc) andpressurized above its critical pressure (Pc), it becomes a supercriticalfluid. Supercritical fluids are known to provide favorable means toachieve solvating properties, which have both gas and liquidcharacteristics without actually changing a chemical structure. Byproper control of pressure and temperature, a significant range ofphysicochemical properties (density, diffusivity, dielectric constants,viscosity, and surface free energy) can be accessed without passingthrough a phase boundary, e.g., changing from gas to liquid form.

As an example, carbon dioxide may exist as a supercritical fluid havingproperties of both a liquid and a gas when above its criticaltemperature (>31° C.) and critical pressure (>7.4 MPa). Carbon dioxideunder supercritical conditions exhibits both a gaseous property, beingable to penetrate through many materials and a liquid property, beingable to dissolve materials into their components. Although carbondioxide is a preferred medium, the supercritical fluid may be selectedfrom other suitable species, such as water, hydrogen peroxide, ozone,water oxidation, methane, ethane, ethylene, or a mixture thereof.

A conventional approach is herein discussed first, which can be used toprepare non-oxidized NGPs. This will be followed by a discussion on aninnovative method developed in our research laboratory. The pristineNGPs prepared will then be subjected to a controlled oxidation treatmentto produce dispersible or soluble NGPs that remain highly conductive. Assuggested by Gulari, et al. [77], one may choose to use a coating agentthat can be solubilized in the supercritical fluid to diffuse betweenthe graphite layers. The purpose of this coating agent, according toGulari, et al. [77], was to allow the coating agent to expand or swellthe interstitial spaces between graphene layers (to assist inintercalation and exfoliation) and, after de-pressurization, the coatingagent will precipitate out to surround and isolate the exfoliatedgraphene platelets. This coating agent (e.g., a polymer) will eventuallybecome a part (the matrix) of a composite material. Generally, thecoating agent may include a polymer, oligomer, monomer, or oil. In oneembodiment, the coating agent is poly-(dimethyl siloxane) (“PDMS”)having a weight average molecular weight of preferably between about30,000 and 200,000 g/mole. Other suitable coating agents includepoly-(tetrafluoroethylene-co-hexafluoropropylene),poly-(perfluoro-propylene oxide), poly-(diethyl-siloxane),poly-(dimethylsilicone), poly-(phenylmethylsilicone),perfluoroalkylpolyethers, chlorotrifluoroethylene, andbromotrifluoroethylene.

The graphitic material particles and the coating agent are preferablyplaced in a compartment of a high pressure vessel isolatable from theatmosphere. In this embodiment, the graphite particles comprise about 23to 83 weight percent and the coating agent comprises about 77 to 17weight percent of material placed in the vessel. The weight ratio ofgraphite particles to coating agent is preferably at least about 1:10.Then, the compartment is sealed off from the atmosphere. The compartmentmay be isolated by any conventional means. This is followed byintroducing high-pressure carbon dioxide into the compartment with CO₂being pressurized in the vessel to preferably above approximately 1,070to 10,000 psig (7.4 MPa to 69 MPa). Then, the vessel is heated to atemperature preferably above about 40° C., and preferably above 70° C.These conditions define a supercritical condition of carbon dioxidewhereby the coating agent is solubilized in the supercritical carbondioxide. Pressurizing and heating the graphitic particles with thesupercritical fluid may be accomplished by any conventional means. Forinstance, the vessel may be heated by a heating jacket or electricalheating tape disposed around the vessel.

With the coating agent being solubilized in the supercritical fluid, thecoating agent diffuses into inter-graphene spaces to possibly expand orswell these spaces. The step of diffusing the coating agent between thegraphene layers includes maintaining diffusion for between about 10minutes to 24 hours (preferably 3 hours) at supercritical conditions toproduce tentatively intercalated graphite.

The procedure further comprises catastrophically depressurizing thetentatively intercalated graphite to precipitate the coating agent fromthe supercritical fluid. During catastrophic depressurization, thesupercritical fluid expands and exfoliates the graphite layers while thecoating agent precipitates from the supercritical fluid to cover thelayers. The depressurization step comprises immediately depressurizingthe vessel down to a considerably lower pressure, preferably ambientpressure. This may be accomplished in a time period of between about 5and 30 seconds, and preferably 15 seconds. This is accomplished bydepressurizing the pressure vessel at a rate of between about 0.1 and5.0 milliliters per second, and preferably 3.0 milliliters per second.The pressure decrease may be accomplished by opening the compartment tothe atmosphere. As immediate depressurization occurs, the graphitelayers are delaminated apart from one another other.

Presumably, the low viscosity and high diffusivity of the supercriticalfluid allows the coating agent solubilized therein to become disposed orintercalated between the graphene layers in the graphitic material undersupercritical conditions, thereby increasing the interlayer spacing.Upon depressurization, the supercritical fluid disposed in theinterstitial spaces force the layers to exfoliate or delaminate fromeach other, and the coating agent previously solubilized in thesupercritical fluid precipitates therefrom to deposit on the delaminatedlayers, preventing reformation of the van der Waals forces betweengraphene layers. That is, the coating agent precipitates from thesupercritical fluid and attaches to the graphene layers.

Although this conventional route is useful in terms of producingpristine NGPs that are covered with a coating agent, one has to removethis coating agent unless the coating agent is desired for an intendedapplication (e.g., for the preparation of a polymer matrix compositewith the coating agent being the monomer or polymer for this matrix).After an extensive study, we have surprisingly observed that:

-   -   (1) Supercritical fluids containing no coating agent are at        least as effective as those containing a coating agent for        intercalating and exfoliating natural graphite. There is no        major difference in the supercritical fluid temperature,        pressure, time, and de-pressurization conditions between the two        species (one with and the other without a coating agent);    -   (2) Supercritical fluids, with or without a coating agent        therein, are effective in intercalating and exfoliating a wide        variety of graphitic materials, including (in addition to        natural graphite) artificial graphite (e.g., highly oriented        pyrolytic graphite, HOPG), graphite oxide, graphite fluoride,        graphite fiber, carbon fiber, carbon nano-fiber, carbon        nano-tube, mesophase carbon micro-bead (MCMB), graphitized soft        carbon, and hard carbon. Previous studies on supercritical fluid        delamination of graphite have been essentially limited to        chemically pre-intercalated natural graphite [Ref. 76] and        natural flake graphite [Ref. 77].    -   (3) With proper conditions for supercritical fluid intercalation        and exfoliation via de-pressurization, one could readily obtain        ultra-thin NGPs with a thickness less than 1 nm. With other less        favorable conditions (e.g., a slower de-pressurization rate),        somewhat thicker NGPs were obtained. However, these thicker NGPs        could be subjected to another cycle of supercritical fluid        intercalation and exfoliation, preferably in the same pressure        chamber, to yield much thinner NGPs. By repeating the process        one or two times we could readily obtain substantially        single-layer NGPs.        Procedure (C):

Another alternative procedure for exfoliating a graphitic material toproduce pristine NGPs comprises (a) dispersing graphitic materialparticles in a liquid medium containing therein a surfactant ordispersing agent to obtain a suspension or slurry; and (b) exposing thesuspension or slurry to ultrasonic waves (a process commonly referred toas ultrasonication) at an energy level for a sufficient length of timeto produce the separated nano-scaled platelets.

Preferably, the ultrasonication step is conducted at a temperature lowerthan 100° C. The energy level is typically greater than 80 watts. Theliquid medium may comprise water, organic solvent, alcohol, a monomer,an oligomer, or a resin. The graphitic material could be naturalgraphite, synthetic graphite, highly oriented pyrolytic graphite,graphite oxide, graphite fiber, graphite nano-fiber, MCMB, soft carbon,hard carbon, or a combination thereof.

It may be noted that ultrasonication has been used to successfullyseparate graphite flakes after thermal exfoliation of chemicallyintercalated graphite. Examples are given in Sakawaki, et al. (“FoliatedFine Graphite Particles and Method for Preparing Same,” U.S. Pat. No.5,330,680, Jul. 19, 1994) and Chen, et al. (“Preparation andCharacterization of Graphite Nanosheets from Ultrasonic PowderingTechnique,” Carbon, Vol. 42, 2004, 753-759). However, there has been noreport on the utilization of ultrasonic waves in directly exfoliatinggraphite or graphite oxide (with or without intercalation) and,concurrently, separating exfoliated particles into isolated or separatedgraphite flakes or platelets with a thickness less than 100 nm. Thisdirect graphite exfoliation procedure was discussed in detail in one ofour earlier inventions [Ref. 75].

Controlled Oxidation of Pristine NGPs

In one preferred embodiment of the present invention, the second step ofthe process involves subjecting the pristine NGPs to a controlledoxidation treatment. As opposed to the original chemicalintercalation/oxidation treatment required in the prior art preparationof graphite oxide nano platelets that involves heavy and essentiallyun-controlled oxidation of natural graphite, the present oxidationprocedure for pristine NGPs has the following advantages: (1) Oxidationcan be executed in a well-controlled manner; (2) The degree of oxidationcan be relatively low (in such a manner that oxidation can be limited tothe edge of NGPs, with the graphene plane surface remainingsubstantially oxygen-free, if so desired); (3) The oxidation procedurecan proceed at a high rate since the original graphitic material hasbeen split into smaller particles and, hence, the oxidizing agent doesnot have to travel through inter-particle regions; and (4) Due to thewell-split and separated nature of NGPs, they can be subjected togaseous phase oxidation, as opposed to liquid phase oxidation thatrequires a post-oxidation cleaning or purification procedure, which istypically very tedious and generates a great amount of waste water.

In one procedure, pristine NGPs may be dispersed in an acid (e.g.,sulfuric acid, nitric acid, carboxylic acid, acetic acid, formic acid,etc.) and/or an oxidizing agent (e.g., KMnO₄, sodium or potassiumchlorate, and hydrogen peroxide, H₂O₂) at a temperature for a desiredperiod of time. More environmentally benign acids or oxidizers, such ascarboxylic acid, acetic acid, formic acid, and hydrogen peroxide, arepreferred. The carboxylic acid may be selected from the group consistingof aromatic carboxylic acid, aliphatic or cyclo-aliphatic carboxylicacid, straight chain or branched chain carboxylic acid, saturated andunsaturated mono-carboxylic acids, di-carboxylic acids andpoly-carboxylic acids that have 1-10 carbon atoms, alkyl esters thereof,and combinations thereof.

Alternatively and preferably, the oxidation treatment comprisessubjecting the pristine NGPs to an oxidizing agent in a vaporous orgaseous state. This oxidizing agent is preferably selected from ozone,sulfonic (SO₃) vapor, an oxygen-containing gas, hydrogen peroxide vapor,nitric acid vapor, or a combination thereof. Further preferably, thetreatment comprises subjecting the pristine NGP material to an oxidizingagent in a hydrogen-containing environment. Hydrogen seems to provideuseful functional groups, such as carboxyl and hydroxyl. Althoughoxidation treatment can be conducted by immersing NGPs in a liquid acidand/or oxidizer environment, such a procedure requires a subsequentwater-rinsing and purification step (such a rinsing procedure is not astedious as required in the case of conventional sulfuricacid-intercalation graphite, nevertheless). Hence, a gaseous treatmentrequiring no post-treatment rinsing is preferred.

A primary goal of the oxidation treatment is to impart solubility ordispersibility to the pristine NGPs without a significant compromise inelectrical conductivity. After an extensive and in-depth study we havediscovered that dispersible and conductive NGPs can be achieved byproducing pristine NGPs first and then imparting to pristine NGPs anoxygen content up to 25% by weight, preferably below 20% by weight,further preferably between 5% and 20% by weight. The oxygen content canbe determined using chemical elemental analysis and/or X-rayphotoelectron spectroscopy (XPS).

It has been hitherto commonly believed by those skilled in the art thatchemical processibility and electrical conductivity of graphitematerials are mutually exclusive. Quite opposite to this common wisdom,we have herein proven that, within a reasonable range of oxygen contentsin NGPs and their associated window of processing conditions, these twofeatures can be achieved at the same time. The good solubility ordispersibility enables the production of NGP-based products, such asgraphene paper, film, and nanocomposite structures, that have desirablephysical properties. The oxygen content, along with some hydrogen, alsoenables us to impart a wide variety of functional groups to the NGPs.

The laminar graphite materials used in the prior art processes for theproduction of the GIC, GO, and subsequently made exfoliated graphite,flexible graphite sheets, and graphene platelets were, in most cases,natural graphite. However, the present invention is not limited tonatural graphite. The starting material may be selected from the groupconsisting of natural graphite, artificial graphite (e.g., highlyoriented pyrolytic graphite, HOPG), graphite oxide, graphite fluoride,graphite fiber, carbon fiber, carbon nano-fiber, carbon nano-tube,mesophase carbon micro-bead (MCMB) or carbonaceous micro-sphere (CMS),soft carbon, hard carbon, and combinations thereof. All of thesematerials contain graphite crystallites that are composed of layers ofgraphene planes stacked or bonded together via van der Waals forces. Innatural graphite, multiple stacks of graphene planes, with the grapheneplane orientation varying from stack to stack, are clustered together.In carbon fibers, the graphene planes are usually oriented along apreferred direction. Generally speaking, soft carbons are carbonaceousmaterials obtained from carbonization of liquid-state, aromaticmolecules. Their aromatic ring or graphene structures are more or lessparallel to one another, enabling further graphitization. Hard carbonsare carbonaceous materials obtained from aromatic solid materials (e.g.,polymers, such as phenolic resin and polyfurfuryl alcohol). Theirgraphene structures are relatively randomly oriented and, hence, furthergraphitization is difficult to achieve even at a temperature higher than2,500° C. But, graphene sheets do exist in these carbons.

Preparation of Graphite Oxide Nano Platelets and their De-Oxygenation

In the second set of approaches, the dispersible and conductive NGPmaterial may be produced by (a) preparing a graphite intercalationcompound (GIC) or graphite oxide (GO) from a laminar graphite material;(b) exposing the GIC or GO to a first temperature for a first period oftime to obtain exfoliated graphite (mostly graphite oxide); and (c)exposing the exfoliated graphite to a second temperature in a protectiveatmosphere for a second period of time (a step called de-oxygenation) toobtain the desired dispersible nano graphene platelet with an oxygencontent between 5% and 25% by weight.

Intercalation or Oxidation of Graphite:

In most of the prior art methods for making separated nano grapheneplatelets, the process begins with intercalating lamellar graphite flakeparticles with an expandable intercalation agent (also known as anintercalant or intercalate) to form a graphite intercalation compound(GIC), typically using a chemical oxidation or an electrochemical (orelectrolytic) method. The GIC is characterized as having intercalantspecies, such as sulfuric acid and nitric acid, residing in interlayerspaces, also referred to as interstitial galleries or interstices. Intraditional GICs, the intercalant species may form a complete or partiallayer in an interlayer space or gallery. If there always exists onegraphene layer between two intercalant layers, the resulting graphite isreferred to as a Stage-1 GIC. If n graphene layers exist between twointercalant layers, we have a Stage-n GIC.

It may be noted that intercalation of graphite (e.g., if intercalated bypotassium melt) does not necessarily lead to oxidation of graphite.However, if the intercalant contains an acid (e.g., sulfuric acid,nitric acid, carboxylic acid, etc.) and/or an oxidizing agent (e.g.,KMnO₄, sodium or potassium chlorate, and hydrogen peroxide, H₂O₂), theresulting GIC is essentially a graphite oxide (GO) material. This istrue of essentially all of the known prior art chemical processes forthe preparation of GO nano platelets.

Exfoliation:

This intercalation or oxidation step is followed by rapidly exposing theGIC or GO material to a high temperature, typically between 800 and1,100° C., to exfoliate the graphite material, forming vermiculargraphite structures known as graphite worms. It is important tounderstand that these graphite worms or their constituent graphiteflakes are actually graphite oxide, not graphene. They typically containmore than 30% by weight of oxygen, existing as oxygen-containingfunctional groups like carboxyl or hydroxyl on both the basal planesurfaces and edges of graphene layers. Exfoliation is believed to becaused by the interlayer volatile gases, created by the thermaldecomposition, phase transition, or chemical reaction of theintercalant, which induce high gas pressures inside the interstices thatpush apart neighboring layers. In some methods, the exfoliation productis graphite worms that contain more or less interconnected graphiteoxide flakes or functional group-decorated graphene sheets that arestill more or less clustered or tied together. In order to furtherseparate these interconnected graphite oxide flakes, the exfoliationproduct may then be subjected to air milling, air jet milling, ballmilling, or ultrasonication before or after the second heat treatment.

In one preferred embodiment of the present invention, a dispersibleNGP-producing process begins with the preparation of a GIC or GOmaterial, followed by heating the GIC or GO material to obtainexfoliated graphite. These two steps are similar to the above-describedtwo steps—intercalation/oxidation of graphite and exfoliation of GIC/GO.Although exfoliation temperature is typically between 800 and 1,100° C.for the GIC or GO prepared from natural graphite, we have found that theGIC or GO prepared from meso-phase carbon micro-beads (MCMB) can beeffectively exfoliated at a temperature as low as 200° C. However, inall cases, higher exfoliation temperatures are preferred and exfoliationis preferably conducted in a protective atmosphere (e.g., containing aninert gas, hydrogen, and/or nitrogen).

It is of significance to note that, in the prior art, for all purposes(e.g., to produce graphite worms, flexible graphite, graphite oxideflakes, or separated graphene oxide sheets), exfoliation of the GIC/GOwas prescribed to occur at a relatively high temperature for a veryshort period of time, typically shorter than 2 minutes, more typicallyshorter than 1 minute, and often shorter than 30 seconds. In the priorart, expansion or exfoliation of graphite oxide was normally completedwithin this short period of time and, hence, continued heating of thefreshly exfoliated graphite was believed to be unnecessary andundesirable (for fear of thermally degrading the exfoliation product orperhaps for the purpose of saving energy).

De-Oxygenation:

Contrary to this conventional wisdom, we have surprisingly observed thata further exposure of the exfoliated graphite product to a hightemperature (typically higher than the exfoliation temperature), but ina protective atmosphere, could de-oxygenate or reduce the graphite oxideplatelets to a range of very unique and useful oxygen contents. Withinthis range, exfoliated graphite oxide platelets become highlyelectrically conducting and yet remain soluble or dispersible in waterand many other organic solvents. In the prior art, dispersibility andconductivity are generally believed to be non-coexisting. This goodsolubility or dispersibility enables the production of NGP-basedproducts, such as graphene paper, film, and nanocomposite structures,that have desirable physical properties. No subsequent chemicalreduction of the platelets is required.

Although partial de-oxygenation of the exfoliated graphite oxide flakeswas suggested by others [e.g., 67, 68] as a means of reducing theproduct to recover electrical properties of nano graphene after theproduct is made (e.g., after graphene oxide thin film or paper isproduced), the prior art tasks [67, 68] were based on chemicalsolution-based GO exfoliation, not thermal exfoliation. However, oncethe de-oxygenation treatment in a vacuum was done, the grapheneplatelets were no longer soluble or dispersible. The prior art has nottaught about the approach of continuing heating or re-heating thethermally exfoliated GO products in a protective atmosphere to obtaindispersible yet conductive NGPs. Furthermore, the prior art has notsuggested that this continual heating or re-heating could be preferablyconducted immediately after, or concurrently with the exfoliation stepto save energy and time. In the presently invented process, furtherpreferably, these two operations (thermal exfoliation andde-oxygenation) are conducted using the same reactor. It has beenhitherto commonly believed by those skilled in the art that chemicalprocessibility and electrical conductivity of graphite materials aremutually exclusive. Quite opposite to this common wisdom, we have hereinproven that, within a reasonable range of oxygen contents in GO nanoplatelets and their associated window of processing conditions, thesetwo features can be achieved at the same time.

The presently invented processes typically resulted in a dispersiblenano graphene platelet material, when formed into a thin film with athickness no greater than 100 nm, exhibits an electrical conductivity ofat least 10 S/cm, often higher than 100 S/cm, and, in many cases, higherthan 1,000 S/cm. The resulting NGP material typically has a specificsurface area of from approximately 300 m²/g to 2,600 m²/g and, in manycases, comprises single-layer graphene sheets.

The process may further comprise a step of contacting the nano grapheneplatelet material, during and/or after oxidation, with a reactant suchthat a functional group is added to a surface or edge of the nanographene platelet material. Theses functional groups may contain alkylor aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group,amine group, sulfonate group (—SO₃H), aldehydic group, quinoidal,fluorocarbon, or a combination thereof.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention.

Examples 1-4 provide first set of samples, based on the preparation ofgraphite oxide (GO) and then de-oxygenation of GO nano platelets.Subsequent examples provide samples that are produced first with thepreparation of pristine NGPs, followed by partial oxidation of pristineNGPs.

Example 1 NGPs from Carbon/Graphite Fibers

Continuous graphite fiber yarns (Magnamite from Hercules) were cut intosegments of 5 mm long and then ball-milled for 24 hours. Approximately20 grams of these milled fibers were immersed in a mixture of 2 L offormic acid and 0.1 L of hydrogen peroxide at 45° C. for 48 hours.Following the chemical oxidation intercalation treatment, the resultingintercalated fibers were washed with water and dried. The resultingproduct is a formic acid-intercalated graphite fiber material containinggraphite oxide crystallites.

Subsequently, approximately ½ of the intercalated or oxidized fibersample was transferred to a furnace pre-set at a temperature of 600° C.for 30 seconds. The compound was found to induce extremely rapid andhigh expansions of graphite crystallites. The as-exfoliated graphitefiber is designated as Sample-1a. Approximately half of Sample 1-amaterial was subjected to de-oxygenation at 1,100° C. for 20 minutes ina nitrogen atmosphere to obtain Sample-1b.

A small amount of both materials was mixed with an aqueous ethanolsolution to form two separate suspensions, which were subjected tofurther separation of exfoliated flakes using a Cowles shearing device.Both graphite oxide platelets (Sample 1-a) and reduced GO platelets(essentially NGPs) were found to be soluble and well-dispersed in thisaqueous solution. The resulting suspensions were dip-coated to form thinfilms with a thickness of approximately 100 nm on glass slide surfaces.The thickness of individual platelets was found to range from twographene sheets to approximately 25 graphene sheets (average of 14sheets or approximately 4.7 nm) based on SEM and TEM observations. Thelength of these NGPs was typically in the range of 10-60 μm and width inthe range of 0.5-2 μm.

A four-point probe method was used to measure theelectrical-conductivity of the thin films on the glass substrate. It wasfound that the conductivity of the film prepared from Sample 1-a(as-exfoliated GO platelets) was approximately 1.3×10⁻³ S/cm while thatof Sample 1-b was 2.8 S/cm.

Example 2 NGPs from Sulfuric Acid Intercalation andExfoliation/De-Oxygenation of MCMBs

MCMB 2528 microbeads were supplied by Alumina Trading, which is the U.S.distributor for the supplier, Osaka Gas Chemical Company of Japan. Thismaterial has a density of about 2.24 g/cm³; a particle size maximum forat least 95% by weight of the particles of 37 microns; median size ofabout 22.5 microns and an inter-planar distance of about 0.336 nm. MCMB2528 (10 grams) were intercalated with an acid solution (sulfuric acid,nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for 24hours. Upon completion of the reaction, the mixture was poured intodeionized water and filtered. The intercalated MCMBs were repeatedlywashed in a 5% solution of HCl to remove most of the sulphate ions. Thesample was then washed repeatedly with deionized water until the pH ofthe filtrate was neutral. The slurry was spray-dried and stored in avacuum oven at 60° C. for 24 hours. The dried powder sample was placedin a quartz tube and inserted into a horizontal tube furnace pre-set ata desired temperature, 600° C. for 30 seconds to obtain Sample 2-a.Approximately one half of the exfoliated MCMB sample was subjected tode-oxygenation treatment at 1,250° C. for 15 minutes in an argonenvironment to obtain Sample 2-b. A small quantity of each sample wasmixed with water and ultrasonicated at a 60 W power for 10 minutes toobtain a suspension. Again, thin films were prepared from eachsuspension by dip coating and the electrical conductivity of the filmswas measured. The conductivity of the film prepared from Sample 2-a(as-exfoliated oxidized MCMB platelets) was found to be approximately1.8×10⁻² S/cm and that of Sample 2-b after de-oxygenation was 67 S/cm.Both types of platelets were well-dispersed in water.

Example 3 Oxidation, Exfoliation, and De-Oxygenation of Natural Graphite

Graphite oxide was prepared by oxidation of graphite flakes withsulfuric acid, sodium nitrate, and potassium permanganate at a ratio of4:1:0.05 at 30° C. for 24 hours, according to the method of Hummers[U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of thereaction, the mixture was poured into deionized water and filtered. Thesample was then washed with 5% HCl solution to remove most of thesulfate ions and residual salt and then repeatedly rinsed with deionizedwater until the pH of the filtrate was approximately 7. The intent wasto remove all sulfuric and nitric acid residue out of graphiteinterstices. The slurry was spray-dried and stored in a vacuum oven at60° C. for 24 hours. The interlayer spacing of the resulting laminargraphite oxide was determined by the Debey-Scherrer X-ray technique tobe approximately 0.73 nm (7.3 Å), indicating that graphite has beenconverted into graphite oxide.

The dried, intercalated (oxidized) compound was divided into twobatches, both for exfoliation at 800° C. for 1 minute by placing thesample in a quartz tube that was inserted into a horizontal tube furnacepre-set at 800° C. For Sample 3-a, exfoliation was followed byde-oxygenation at 1,000° C. for various periods of time, from 1 minuteto 120 minutes. For Sample 3-b, the de-oxygenation temperature was1,100° C., from 1 minute to 80 minutes. The de-oxygenation atmospherewas approximately 95% nitrogen and 5% hydrogen.

Two series of thin films were prepared from these two samples for thepurpose of measuring the electrical conductivity of the GO nanoplatelets or NGPs as a function of the de-oxygenation time and theresulting oxygen content. The oxygen content, based on the elementalanalysis, was for those oxygen atoms in functional groups attached tothe plane surfaces and edges of the platelets. The exfoliated andde-oxygenated products, after various periods of de-oxygenation, wereeach mixed with water and then subjected to a mechanical shearingtreatment using a Cowles rotating-blade shearing machine for 20 minutes.The resulting platelets were found to have an average thickness of 6.3nm. Spin coating was used to prepare thin films for conductivitymeasurement. GO or graphene platelets at selected de-oxygenation timeintervals were also analyzed for their oxygen contents using X-rayphotoelectron spectroscopy (XPS) available at the Center forMultifunctional Nonmaterial at Wright State University, Dayton, Ohio.

Shown in FIG. 1 is a summary of the electrical conductivity data of thefilms made from GO nano platelets after various periods ofde-oxygenation time at 1,000° C. and 1,100° C., respectively. Theconductivity of the film varies from 5.0×10⁻³S/cm of as-foliated GO to180 S/cm after 40 minutes of de-oxygenation, and to 4.1×10² S/cm after80 minutes, the latter representing a five order-of-magnitudeimprovement in electrical conductivity. The GO or de-oxygenated GOplatelets were found to be soluble or dispersible in water up to anoxygen content of 5.6% by weight (after 50 minutes at 1,100° C., givingrise to an electrical conductivity of 360 S/cm). This conductivity valueis a very impressive result, comparable to the best achievableconductivity with strong or heavy chemical reduction and/or vacuumde-oxygenation treatments after the films were made (yet those grapheneplatelets of the thin films prepared in the prior art becamenon-dispersible) [Refs. 47, 67, 68].

The two curves and the observations made on the solution dispersibilityof the corresponding suspensions appear to indicate that theconductivity increases rapidly with the degree of de-oxygenation whilethe GO platelets remain soluble over a range of treatment time durationsat a given de-oxygenation temperature; e.g., up to 50 minutes at 1,100°C. Once the conductivity value reaches a plateau, the platelets begin tolose their solubility or dispersibility in water and other polarsolvents, such as ethanol and acetone. Fortunately, this plateau valueis already very high, typically in the range of 100-1,000 S/cm.

The electrical conductivity data were plotted as a function of thecorresponding oxygen content data for two de-oxygenation temperatures,as shown in FIG. 2. It is clear that, regardless of the de-oxygenationtemperature, it is the final oxygen content that governs theconductivity of GO or reduced GO platelets; the lower the oxygencontent, the higher the conductivity is. When the oxygen content isbelow 5% by weight, the reduced GO tends to become insoluble ornon-dispersible in water. Surprisingly, and fortunately, within theoxygen content range of 5%-20%, the nano platelet film exhibits aconductivity value greater than 1 S/cm. If the oxygen content is below15%, the conductivity is greater than 10 S/cm. The conductivity of theNGP film is greater than 100 S/cm if the oxygen content is below 10%.

Example 4 Oxidation, Exfoliation, De-Oxygenation, and FurtherFunctionalization of Natural Graphite

The samples of Example 4, including Sample 4-a and 4-b, were prepared ina similar manner as described in Example 3, but the exfoliation wasconducted at 1,000° C. for 45 seconds, followed by de-oxygenation at1,200° C. and 1,350° C., respectively, for various periods of time.Shown in FIG. 3 is a summary of the electrical conductivity data of thefilms made from GO nano platelets after various periods ofde-oxygenation time. These data further confirm the trend observedearlier that the electrical conductivity of nano graphene or grapheneoxide films increases with increasing de-oxygenation time (or decreasingoxygen content). High conductivity can be attained with shorter periodsof time if the de-oxygenation temperature is sufficiently high.

In order to determine if a lower oxygen content would adversely affectthe functionalization capability of graphene platelets and howfunctionalization would impact the electrical conductivity of theseplatelets, we carried out additional work on selected samples, describedbelow: With the de-oxygenation atmosphere containing some hydrogen, wepresumed that the edges of graphene or graphene oxide plateletscontained a significant amount of activated C—H bonds. We chose tosulfonate the two samples that had been de-oxygenated for 10 minutes and45 minutes, respectively, at 1,200° C. The sample with a 10-minde-oxygenation treatment (Sample 4-a-10) was highly soluble in water,but that with a 45-minute treatment (Sample 4-a-45) has poor or limitedsolubility in water. Sulfonation was conducted by subjecting the twosamples to the vapor phase of a fuming sulfuric acid (oleum) containing20% SO₃ for one hour. The results were very surprising. After thesulfonation treatment, Sample 4-a-10 remained highly soluble in waterand Sample 4-a-45, originally having limited solubility, became solublein water. Most surprisingly, the electrical conductivity of theirrespective films remained essentially un-changed, 12 S/cm and 695 S/cm,respectively. This important observation suggests that furtherfunctionalization of de-oxygenated graphene platelets provides anothertool of varying solubility of the graphene platelets, as prepared by thepresently invented de-oxygenation process, without adversely affectingtheir conductivity.

Sulfonation is but one of many approaches to the functionalization ofde-oxygenated GO platelets. Presumably, both the functional groupsattached to basal plane atoms and those at the edges of basal planes (orgraphene planes) tend to decrease the electrical conductivity of agraphene or graphene oxide platelet. The surface functional groups arein the way of electron conduction paths and, hence, are much moreinfluential on the electron transport. These groups represent defectsthat could significantly reduce the mean free path of electrons movingon a basal plane. The functional groups at the graphene edge, althoughaltering the quantum wave functions of electrons at the edge, would haveless significant effect on the overall conductivity. However, thepresence of different functional groups could have significantlydifferent effects on solubility or dispersibility of a graphene orgraphene oxide platelet in a solvent and the interfacial bonding betweena platelet and a matrix material in a nanocomposite. This implies thatwe now have a tool of adjusting the solubility or dispersibility of NGPsin a solvent without significantly varying the electrical conductivity.

Example 5 Preparation of Pristine NGPs from Natural Flake Graphite andGraphite Fibers Using Direct Ultrasonication

Five grams of graphite flakes, ground to approximately 20 μm or less insizes, were dispersed in 1,000 mL of deionized water (containing 0.1% byweight of a dispersing agent, Zonyl® FSO from DuPont) to obtain asuspension. An ultrasonic energy level of 75 W (Branson S450Ultrasonicator) was used for exfoliation, separation, and size reductionfor a period of 1 hour. This procedure was repeated several times, eachtime with five grams of starting graphite powder, to produce asufficient quantity of pristine NGPs, which are hereinafter referred toas Sample 5-p-U (p stands for pristine and U stands for ultrasonic).

The same procedure was repeated for graphite fibers to obtain Sample 5Gf-p-U. The graphite fibers used were the same as those in Example 1.

Example 6 Preparation of Pristine NGPs from Natural Graphite Flakes andMCMBs Using Potassium Intercalation

Natural graphite was obtained from Huadong Graphite Co., Qingdao, China.The first stage intercalation compound, KC₈, was synthesized by adding astoichiometric amount of potassium, 81.4 mg (0.0021 moles) to 200 mg(0.0167 moles) of graphite particles in a Pyrex tube capped with astopcock. All transfers were carried out in a helium filled dry box. Thereactant filled tube was evacuated, sealed and heated for 16 hours at200° C. The compound formed was bright gold in color. The obtained GICwas poured into a mixture of ethanol and distilled water (50:50 byvolume). The material turns from gold to black as the graphite gotexfoliated and bubbling was observed, suggesting that hydrogen wasproduced. The resulting solution was basic due to the formation ofpotassium ethoxide. The dispersion of nano graphene sheets in aqueousethanol solution was then allowed to settle. The solvent was decantedand the product washed several times with ethanol until a neutral pH wasobtained. This pristine NGP material is referred to as Sample 6-p-K (Kstands for potassium intercalation).

Another batch of samples was prepared from MCMBs following the sameprocedures. This pristine NGP material is referred to as Sample 6MC-p-K.

Example 7 Preparation of Pristine NGPs Using Supercritical Fluids

A natural graphite sample (approximately 5 grams) was placed in a 100milliliter high-pressure vessel. The vessel was equipped with securityclamps and rings that enable isolation of the vessel interior from theatmosphere. The vessel was in fluid communication with high-pressurecarbon dioxide by way of piping means and limited by valves. A heatingjacket was disposed around the vessel to achieve and maintain thecritical temperature of carbon dioxide.

High-pressure carbon dioxide was introduced into the vessel andmaintained at approximately 1,100 psig (7.58 MPa). Subsequently, thevessel was heated to about 70° C. at which the supercritical conditionsof carbon dioxide were achieved and maintained for about 3 hours,allowing carbon dioxide to diffuse into inter-graphene spaces. Then, thevessel was immediately depressurized “catastrophically’ at a rate ofabout 3 milliliters per second. This was accomplished by opening aconnected blow-off valve of the vessel. As a result, delaminated orexfoliated graphene layers were formed, which were identified as Sample7-p-SC-A. This sample was found to contain pristine NGPs with an averagethickness just under 10 nm.

Approximately two-thirds of Sample 7-p-SC-A was removed from thepressure vessel. The remaining NGPs were subjected to another cycle ofsupercritical CO₂ intercalation and de-pressurization treatments (i.e.,the above procedures were repeated), yielding much thinner NGPs with anaverage thickness of 2.1 nm (identified as Sample 7-p-SC-B). Thespecific surface area, as measured by the BET method, was approximately430 m²/g. TEM and AFM examinations indicated that there were manysingle-layer graphene sheets in this sample.

Sample 7-p-SC-C was prepared under essentially identical supercriticalCO₂ conditions, with the exception that a small amount of surfactant(approximately 0.05 grams of Zonyl® FSO) was mixed with 5 grams ofnatural graphite before the mixture was sealed in the pressure vessel.The resulting NGPs have a surprisingly low average thickness, 3.1 nm.After the pressurization and de-pressurization procedures were repeatedfor one cycle, the resulting NGPs have an average thickness less than 1nm, indicating that a majority of the NGPs are single-layer ordouble-layer sheets. The specific surface area of this sample after arepeated cycle was approximately 900 m²/g. It may be noted that a sampleof single-layer graphene sheets should exhibit a specific surface areaof >2,670 m²/g. It is clear that the presence of a surfactant ordispersing agent promotes separation of graphene layers, perhaps bypreventing the reformation of van der Waals forces between graphenesheets once separated.

Example 8 Controlled Oxidation of Pristine NGPs

The oxidation treatment of the samples prepared in Examples 5-7 wascarried out in two different ways: one in a liquid state and the otherin a vapor/gas state.

As one example for vapor phase oxidation, SO₃ vapor was generated byadding and heating 10 g of fuming sulfuric acid into a reactor The SO₃vapor was passed through a column in which 10 g of pristine NGPs waspacked for the oxidation treatment for various durations of time, from 5minutes to one hour. Upon completion of the oxidation, the columncontaining oxidized NGPs was slightly heated at about 60° C. for about15 minutes to remove excessive amount of SO₃ condensed on the surface ofthe NGPs, and the separated SO₃ was recovered and absorbed into thesulfuric acid in the reactor. SO₃-treated NGPs were washed with waterand filtered.

Another example for vapor/gas phase oxidation entails simply exposingNGPs to a heated oxygen or oxygen-containing gas environment (e.g.,oxygen gas with a small amount of water, 1% by weight). A suitabletemperature range is 250-500° C. for 10 to 120 minutes. It may be notedthat graphite oxidation was generally believed to occur only at atemperature higher than 350° C. However, much to our surprise, oxidationof graphene could occur at as low as 200° C. This perhaps is due to thenano-scaled nature of NGPs.

Liquid state oxidation of pristine NGPs can be carried out by simplyimmersing NGPs in a solution containing an acid and/or oxidizer. Morebenign acids, such as acetic acid and formic acid, are preferredalthough it takes a longer period of time to complete an oxidationprocedure as compared with sulfuric or nitric acid. More benignoxidizers, such as H₂O₂, are preferred over other strong oxidizers, suchas potassium permanganate. It was more difficult to wash and remove asalt like potassium permanganate from treated NGPs.

Oxidation conditions of a wide range of pristine NGPs are summarized inTable 1.

TABLE 1 Properties of partially oxidized NGPs. Pristine NGP Solubilityor Oxygen Contents Sample Oxidation Conditions Dispersibility (wt. %)5-p-U SO₃ vapor at 80° C. for 5 to 60 Soluble in water or 3.5-18.5%minutes alcohol when O content >5% by wt 5Gf-p-U SO₃ vapor at 80° C. for5 to 60 Soluble in water or 3.9-21.5% minutes alcohol when O content >5%by wt 6-p-K O₂ + 1% water vapor at 350° C. for Soluble in water or4.6-24.5% 10-80 minutes alcohol when O content >5% by wt 6MC-p-K O₂ at350° C. for 10-80 minutes Soluble in NMP, 2.8-27% toluene, acetone7-p-SC-A Nitric acid at 45° C. for 5-80 Soluble in water or 5.2-34%minutes alcohol when O content >5% by wt 7-p-SC-B Formic acid at 45° C.for 15-120 Soluble in water or 3.8-28% minutes alcohol when Ocontent >5% by wt 7-p-SC-C Aqueous H₂O₂ (30%) solution at Soluble inNMP, 5.5-31.3% 45° C. for 10-120 minutes toluene, acetone

There are two significant observations that can be made from Table 1:(1) Typically, NGPs become soluble in polar solvents, such as water andalcohol, when the oxygen content exceeds 5% by weight; and (2) Some ofthe NGPs, after partial oxidation, actually become soluble in non-polarsolvents, such as NMP and toluene, which is an un-expected result. Thisis surprising since a higher oxygen content means the presence of morepolar groups, such as carboxyl and hydroxyl, that make the NGPs morepolar.

Pristine and partially oxidized NGPs from selected samples (6MC-p-K,7-p-SC-A, 7-p-SC-B, and 7-p-SC-C) were made into thin films(approximately 100 nm thick) for electrical conductivity measurements.The results are summarized in FIG. 4, along with the conductivity dataof those NGPs or GO prepared from de-oxygenating heavily oxidized GOplatelets. It is clear that the electrical conductivity of NGPs or GOnano platelets decreases with increasing oxygen content, regardless ofthe preparation routes.

For those samples prepared from oxidation of pristine NGPs, theconductivity is normally greater than 1 S/cm if the oxygen content isless than 25% by weight, similar to the case of partially de-oxygenatedGN platelets. However, the electrical conductivity of those prepared bypartially oxidizing pristine NGPs is almost always greater than that ofthose prepared by partially de-oxygenating GO nano platelets with acomparable oxygen content. This is another highly surprising result ofthe present study. We speculated that those heavily oxidized GOplatelets were highly defected, likely with their graphene planepopulated with oxygen-containing chemical groups. Even after partialde-oxygenation, these original chemical group sites remain as structuraldefects. By contrast, oxidation of pristine NGPs might begin with thegraphene edges and chemical groups began to attach to the graphenesurface only after the edges were fully oxidized.

Example 9 Various Surface Functionalization Treatments of PartiallyOxidized NGPs

The partially oxidized NGPs prepared according to a preferred embodimentof the present invention can be further functionalized by carrying outan additional step of contacting the partially oxidized NGPs with areactant such that a functional group is added to a surface or edge ofthe nano graphene platelet. The functional group may be selected from,as examples, alkyl or aryl silane, alkyl or aralkyl group, hydroxylgroup, amine group, fluorocarbon, or a combination thereof.

As an example, two separate batches of Sample 6-p-K were subjected tooxidation treatments to obtain Sample 6-p-K-4.6 (oxygen content of 4.6%by weight) and Sample 6-p-K-24.5 (oxygen content of 24.5% by weight).They were then allowed to undergo various functionalization treatments,briefly described as follows:

The NGPs, after a partial oxidation treatment, will have a reactivegraphene surface (RGS) or reactive graphene edge (RGE). They wereprescribed to undergo the following reactions:

(a) RGS/RGE+CH₂═CHCOX (at 1,000° C.)→Graphene-R′COH (where X=—OH, —Cl,—NH₂, or —H); e.g., RGS/RGE+CH₂═CHCOOH→G-R′CO—OH (where G=graphene);

(b) RGS/RGE+Maleic anhydride→G-R′(COOH)₂;

(c) RGS/RGE+Cyonogen→G-CN;

(d) RGS/RGE+CH₂═CH—CH₂X→G-R′CH₂X (where X=—OH, -halogen, or —NH₂);

(e) RGS/RGE+H₂O→G=O (Quinoidal);

(f) RGS/RGE+CH₂═CHCHO→G-R′CHO (Aldehydic);

(g) RGS/RGE+CH₂═CH—CN→G-R′CN;

In the above-listed reactions, R′ is a hydrocarbon radical (alkyl,cycloalkyl, etc).

The results of electrical conductivity measurements of the NGP films andobservations on solubility of NGPs in solvents are summarized in Table2. These data further confirm that chemical functionalization treatmentscan be used to vary the solubility or dispersibility of NGPs withoutsignificantly compromising electrical conductivity.

TABLE 2 Conductivity and solubility of functionalized NGPs.Functionalization Thin Film Electrical Sample Treatment Conductivity(S/cm) Solubility in a Solvent 6-p-K-24.5 None 0.9 Highly soluble inwater, acetone, ethanol, etc. 6-p-K-4.6 None 1,036 Limited solubility inwater, acetone, ethanol 6-p-K-4.6 Reaction (a), X = —OH 988 Becamesoluble in water and ethanol 6-p-K-4.6 Reaction (b) 983 Became solublein water and ethanol 6-p-K-24.5 Reaction (c) 0.8 Highly soluble inwater, acetone, ethanol, etc. 6-p-K-4.6 Reaction (d), X = —NH₂ 985Became soluble in acetone 6-p-K-24.5 Reaction (e) 1.1 Highly soluble inwater, acetone, ethanol, etc. 6-p-K-24.5 Reaction (e) 1.0 Highly solublein water, acetone, ethanol, etc. 6-p-K-24.5 Reaction (f) 0.9 Highlysoluble in water, acetone, ethanol, etc.

Example 10 Functionalization or Derivatization of Partially OxidizedNGPs

Partial oxidation of pristine NGPs can lead to the attachment of somefunctional groups on a surface or at an edge of a graphene plane,including carboxylic acid and hydroxyl groups. A large number ofderivatives can be prepared from carboxylic acid alone. For instance,alcohols or amines can be easily linked to acid to provide stable estersor amides. If the alcohol or amine is part of a di- or poly-functionalmolecule, then linkage through the O— or NH— leaves the other functionalgroup(s) as pendant group(s). For instance, we can have R—OH or R—NH₂,where R=alkyl, aralkyl, aryl, fluoroethanol, polymer, and SiR₁₃.Examples include C₁—SiR₁₃, HO—R—OH (R=alkyl, aralkyl, or CH₂O—),H₂N—R—N₂H(R=alkyl, aralkyl), X—R—Y (R=alkyl, etc.; X═OH or NH₂; Y═SH,CN, C═O, CHO, alkene, alkyne, aromatic, or heterocycles).

As an example, Sample 6-p-K-24.5 was treated to undergo the followingreactions: R—COOH+Im-CO-Im→R—CO-Im+Him+CO₂ (Im=imidazolide) andHim=imidazole), which was followed by R—CO-Im+R′OH (inNaOEt)→R—CO—OR′+Him, and, separately for another specimen, byR—CO-Im+R′NH₂→R—CO—NHR′+Him.

In summary, the presently invented NGP compositions and relatedprocesses are superior to many prior art pristine graphene or graphiteoxide nano platelets and their processes in several aspects:

-   -   1) Prior art processes based on graphite intercalation/oxidation        and high-temperature exfoliation did not allow for a good        control over the oxygen content of the resulting GO or NGP        platelets.    -   2) In another commonly used prior art approach, the graphite        oxide dispersed in an aqueous solution was reduced with        hydrazine, in the presence of a polymer, such as poly (sodium        4-styrenesulfonate). This process led to the formation of a        stable aqueous dispersion of polymer-coated graphene platelets.        In some applications of NGPs, however, a polymer coating may be        undesirable. Furthermore, the reducing agent, hydrazine, is a        toxic substance.    -   3) Another prior art method of producing less hydrophilic GO        platelets involved using an isocyanate treatment. However,        unless stabilized by selected polymers, the chemically modified        graphene sheets obtained through this method tended to        precipitate as irreversible agglomerates due to their        hydrophobic nature. The resulting agglomerates became insoluble        in water and organic solvents. By contrast, the presently        invented process provides a simple and convenient approach to        the preparation of soluble or dispersible nano graphene that is        also highly conducting.    -   4) Conventional processes of preparing GO nano sheets that        included chemical exfoliation typically were extremely tedious.        Such a long process is not amenable to the mass production of        conductive nano graphene platelets. In these prior art        processes, by subjecting the GO films to a high temperature        treatment in a vacuum, one could obtain nano platelets with thin        film electrical conductivity as high as 550 S/cm. However, once        such a high temperature treatment was done, the GO nano sheets        were no longer dispersible in water.    -   5) The presently invented processes are capable of producing        NGPs with no or little impurity. The process can obviate the        need for washing and rinsing the platelets (which was required        in the prior art solution approach to the exfoliation of GO        and/or subsequent chemical reduction).    -   6) The presently invented processes allow for the NGPs to be        readily or easily functionalized. This is particularly useful if        NGPs are used as a filler in a composite material. Solubility or        dispersibility of NGPs in a solvent allows mixing of these NGPs        with a polymer that is also soluble in the same solvent.    -   7) The presently invented processes enable us to have separate        control over dispersibility and conductivity, which were        considered mutually exclusive in the prior art.

The invention claimed is:
 1. A dispersible and electrically conductivenano graphene platelet (NGP) material comprising at least a single-layeror multiple-layer graphene sheet, wherein said NGP material has anoxygen content no greater than 25% by weight and no less than 5% byweight, wherein said nano graphene platelet material is obtained from aprocess comprising: (a) preparing a pristine NGP material from agraphitic material; and (b) subjecting said pristine NGP material to anoxidation treatment to obtain said dispersible and conductive NGPmaterial and wherein said graphitic material is selected from the groupconsisting of natural graphite, artificial graphite, graphite oxide,graphite fluoride, graphite fiber, carbon fiber, carbon nano-fiber,carbon nano-tube, mesophase carbon micro-bead (MCMB) or carbonaceousmicro-sphere (CMS), soft carbon, hard carbon, and combinations thereof.2. The material as set forth in claim 1 wherein said NGP has an oxygencontent in the range of approximately 5% to 20% by weight inclusive. 3.The material as set forth in claim 1 wherein said nano graphene plateletmaterial, when formed into a thin film with a thickness no greater than100 nm, exhibits an electrical conductivity of at least 10 S/cm.
 4. Thematerial as set forth in claim 1 wherein said nano graphene plateletmaterial, when formed into a thin film with a thickness no greater than100 nm, exhibits an electrical conductivity of at least 100 S/cm.
 5. Thematerial as set forth in claim 1 wherein said nano graphene plateletmaterial, when formed into a thin film with a thickness no greater than100 nm, exhibits an electrical conductivity of at least 1,000 S/cm. 6.The material as set forth in claim 1 wherein said nano graphene materialhas a specific surface area of approximately from 300 m²/g to 2,600m²/g.
 7. The material as set forth in claim 1 wherein said nano grapheneplatelet material comprises single-layer graphene.
 8. The material asset forth in claim 1 wherein said oxidation treatment comprisessubjecting said pristine NGP material to an oxidizing agent selectedfrom ozone, sulfonic vapor (SO₃), an oxygen-containing gas, hydrogenperoxide vapor, nitric acid vapor, or a combination thereof.
 9. Thematerial as set forth in claim 1 wherein said pristine NGP material isobtained from a process comprising a procedure selected from: a.intercalating said graphitic material with a non-oxidizing agent,followed by a thermal or chemical exfoliation treatment in anon-oxidizing environment; b. subjecting said graphitic material to asupercritical fluid environment for inter-graphene layer penetration andexfoliation; c. dispersing said graphitic material in a powder form toan aqueous solution containing a surfactant or dispersing agent toobtain a suspension; and subjecting said suspension to directultrasonication; or a combination thereof.
 10. The material as set forthin claim 1, further comprising a functional group selected from alkyl oraryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group,amine group, aldehydic group, quinoidal, fluorocarbon, or a combinationthereof.
 11. A dispersible and electrically conductive nano grapheneplatelet (NGP) material comprising at least a single-layer ormultiple-layer graphene sheet, wherein said NGP material has an oxygencontent no greater than 25% by weight and no less than 5% by weight andsaid NGP further comprises a functional group attached to a surface oredge of the nano graphene platelet material, wherein said functionalgroup is selected from alkyl or aryl silane, alkyl or aralkyl group,hydroxyl group, carboxyl group, amine group, aldehydic group, quinoidal,fluorocarbon, or a combination thereof.
 12. A dispersible andelectrically conductive nano graphene platelet (NGP) material comprisingat least a single-layer or multiple-layer graphene sheet, wherein saidNGP material has an oxygen content no greater than 25% by weight and noless than 5% by weight, wherein said NGP material is obtained from aprocess comprising: (a) preparing a graphite intercalation compound(GIC) or graphite oxide (GO) from a laminar graphite material; (b)exposing said GIC or GO to a first temperature for a first period oftime to obtain exfoliated graphite; and (c) exposing said exfoliatedgraphite to a second temperature in a protective atmosphere for a secondperiod of time, and wherein said nano graphene platelet material, whenformed into a thin film with a thickness no greater than 100 nm,exhibits an electrical conductivity of at least 100 S/cm.
 13. Thematerial as defined in claim 12, wherein said protective atmospherecomprises an inert gas, nitrogen, hydrogen, a combination thereof, orvacuum.
 14. The material as defined in claim 12, wherein said firsttemperature is between approximately 200° C. and 1,500° C.
 15. Thematerial as set forth in claim 12 wherein said first temperature isbetween approximately 800° C. and 1,300° C.
 16. The material as setforth in claim 12 wherein said second temperature is betweenapproximately 800° C. and 1,500° C.
 17. The material as set forth inclaim 12 wherein said second temperature is at least 1,000° C.
 18. Thematerial as set forth in claim 12 wherein said second temperature is atleast 1,200° C. and the second period of time is at least 2 minutes.